1. Field of the Invention
This invention relates to a surface treatment of a metal material by an electric discharge. For example, it relates to an electric discharge surface treating method and apparatus for forming a coating layer on a workpiece metal surface by generating the electric discharge between an electrode and the workpiece. The electrode may be made of a reforming material or its raw material. Otherwise, the electrode is formed of a commonly used material. This invention is applicable to a surface treatment of a tool or metal mold as well as a surface treatment of an object which needs a corrosion resistance and a wear resistance such as a machine structure and machine parts. Moreover, this invention is also relates to a surface treatment for providing a finished surface roughness of high quality as well as a surface coating with a high wear resistance on a surface of a steel material or a hard metal. The hard metal may be a sintered metal of tungsten carbide-cobalt.
2. Description of the Related Art
It is conventional to give a corrosion resistance and wear resistance by forming a coating on a surface of a metal material by an electric discharge in a working fluid.
For example, an electric discharge is generated in a working fluid between a workpiece and an electrode which is formed by mixing and compressing powders of WC (tungsten carbide) and Co (cobalt) as an electrode material. The electrode material is deposited on the workpiece to define a coating layer. Then, a remelting electric discharge machining (EDM) is performed by another electrode such as Cu (copper) electrode or Gr (graphite) electrode. Thus, it is possible to give the coating layer higher hardness and adhesion.
Moreover, nitriding is known to heighten strength of a steel or the like.
For example, a workpiece such as a forging die is processed by nitriding after cut by a tool or shaped by an electric discharge. Before the nitriding, a treated surface of the workpiece is polished, because nitrogen is hard to go into the electrically discharged surface as such. Therefore, if the workpiece undergoes a heat treatment such as quench hardening or the like after the nitriding, a hardened structure does not easily return to its previous state by a high temperature of such heating operation.
Next, a conventional art will be described in detail referring to FIGS. 15a-15c.
FIG. 15a is an explanatory drawing showing a first processing in a conventional electric discharge surface treating method. FIG. 15b is an explanatory drawing showing a second processing in the conventional electric discharge surface treating method. FIG. 15c is a block diagram showing the conventional electric discharge surface treating method.
In the first processing, a workpiece of a base metal S50C is machined by electric discharge in a working fluid by use of a green compact electrode mixed with WC--Co, so that WC--Co is deposited on the workpiece. In the second processing, a surface of the workpiece is remelted by an electrode such as a Cu electrode which is hard to wear. When WC--Co is deposited on the workpiece in the first processing, the hardness of the coating layer structure is about Hv=1410 and many voids are remained thereat. However, when remelting WC--Co layer in the second processing, the voids of the coating layer disappear and the hardness is heightened up to Hv=1750.
Such surface treating method can provide a steel material with a coating layer which has a high hardness and good adhesion. However, it is difficult to form such a coating layer of good adhesion on a surface of a sintered material such as a hard metal.
As mentioned above, there are some kinds of workpiece which is hard to get a coating layer of high quality by a conventional ED (electric discharge) surface treating method.
The inventors of the present application made an experiment as follows. An EDM was conducted in oil by using an electrode made by compacting a metal hydride such as TiH2 (titanium hydride). Then, the oil was decomposed to produce carbon by high temperature in the electric discharge, thereby composing TiC (titanium carbide). Moreover, TiH2 was decomposed to produce hydrogen. Such hydrogen cleaned a coating surface. As a result, it was confirmed that a surface coating layer of high strength and good adhesion could be formed thereon. It was also confirmed that, in case of using TiH2, the coating layer was constructed of carbonized TiC and non-carbonized Ti or their intermediate product. In case of using VH or the like instead of TiH2, the same result was obtained, too. If V (vanadium), VC or the like was added to TiH2, the coating layer could be given much higher hardness. In this way, if the EDM is performed in oil by the electrode of compacted metal hydride, the coating layer has high hardness and shows high wear resistance in most cases (in normal abrasion tests or the like).
However, a cutting tool edge or a cold forging die is applied with a high pressure from a metal workpiece material and sometimes heated thereby. Then, an affinity is generated between the workpiece and a coating layer surface by the electric discharge, i.e. a surface of the cutting tool edge and the like. Thus, an abrasion amount is increased, so that the life of the cutting tool or the die cannot be as long as expected from the high hardness and high wear resistance.
In case of an electric discharge surface treating using a green compact electrode, a finished surface roughness of a workpiece becomes large if it is desired to heighten a surface treating rate. At present, the best finished surface roughness is about 6 .mu.mRz for a hard metal workpiece and about 9 .mu.mRz for a steel workpiece, on condition that the surface treating rate is relatively high. A finished surface roughness of each of the workpieces is about 1 .mu.mRz or less before the surface treating. Thus, the surface roughness is deteriorated and enlarged by the electric discharge surface treatment.
This is because the green compact electrode is worn during the electric discharge surface treating, thereby producing irregularity thereon. Moreover, material grains of the green compact electrode such as titanium hydride (TiH2) are hard to be broken into fine powders. Fine powdering may cause ignition and explosion or the like when pulverized. Furthermore, the electric discharge may partially concentrate due to uneven electric resistance of the green compact electrode.
In the electric discharge surface treatment, melted coating material of high temperature hits and collides with the workpiece to be scattered thereon. Thus, the ED surface treatment is advantageous compared with the PVD (physical vapor deposition), CVD (chemical vapor deposition), plating or the like, in view of its very strong adhesion. However, as mentioned above, it is hard to get a finished surface roughness up to 1 .mu.mRz by the ED surface treatment.
The EDM can be used for a surface treatment of common wear resisting parts. However, it is not suitable for a surface treatment of a workpiece which needs a very fine finished surface roughness of about 1 .mu.mRz. For example, the EDM is not preferred for cutting tools, cold forging dies, metal molds, or mechanical parts used in a severe condition such as bearings, construction and building machines and ship's parts.